High strength cost steel for use at cryogenic temperatures

ABSTRACT

An alloy steel containing, among others, nickel, manganese, molybdenum, carbon and an element such as calcium, offers relatively high strength and excellent toughness at cryogenic temperatures.

Unlted States Patent 1 1 1111 3,811,873 deBarbadillo, II 1 May 21, 1974[54] HIGH STRENGTH LOW COST STEEL FOR I 3,671,336 6/1972 Korchynsky eta1 148/36 2,204,585 6/1940 Gagnebln 75/123 R USE AT CRYOGENICTEMPERiTURES 2,451,469 10/1948 Brophy et a1. 1. 75/123 R {75] vInventor: John Joseph deBa b oy 2,992,143 7/1961 Yeo et a1. 75/123 KWarwick, N.Y. 3,388,988 6/1968 Nagashimo et all 148/36 X v 1 3,444,0115/1969 N 11" 1 1... 148/36 x [731 Asslgneei The lmematlona' Nlckel p3,620,717 11/197'1 5315111 6 75/123 K 1119-, w York, NY 3,655,36610/1969 136191111. 75/123 K [22] Filed: Jan. 31, 1972 OTHER PUBLICATIONS21 A 1. No.1222021 Metallurgical Transactions, V01. 1, December 1970, [1pp 33413350Khkt1 pgs. orc yns y e a. [52] US. Cl 75/123 K, 75/123 R,148/36 v 511 1m. (:1 C22C 39/36, C22C 39/54 Examm Charles [58] Field ofSearch 75/123 R, ABSTRACT An alloy steel containing, among others,nickel, man- 5 3 R f r c Ci ganese, molybdenum, carbcn and an elementsuch as UNITED STATES PATENTS calclltllm, offers relat vely highstrength and excellent toug ness at cryogemc temperatures. 3,619,30211/1971 -Aok1eta|. 148/-'12 2,258,604 10/1941 Gagnebin 75/123 R 2Claims, 1 Drawing Figure g g ls- 0 v 1 1 1 1 1 T 4.5 610 56 6.0 65 20 26a0 1 HIGH STRENGTH-LOW COST STEEL FOR USE AT CRYOGENIC TEMPERATURES Thesubject invention-is addressed'primarily, but not exclusively,'to theproblem of producing an economical steel characterized by tensilestrength and impact toughness of such magnitude as to render the steelsuitable for use at cryogenic temperatures as low as 320F., suchproperties being particularly useful in the production of storagevessels for liquified gases.

The processing and storage of liquified gases presents many problems,one of which is the selection of the most economical material ofconstruction. At present, the so-called 8 percent and 9 percent nickelsteels are being extensively used in these applications, with thespecifications for these materials being set forth in ASTM Designation:A 553-70a. As is shown in the 9 percent'nickel steel US. Pat. to Brophyand Miller, No. 2,451,469, cryogenic toughness is dependent on nickelcontent. This is demonstrated by comparing the 9 percent nickelsteel,wl1ich is suitable for temperatures as low as -320F., and the 8percent nickel steel, which is limited to temperatures above 2' 7 5F.This temperature difference issignificant since for many importantindustrial materials, such as LNG, tank fabricators require the steel tomeet the specifications for temperatures as low as 320F.

With the objective .of developing lower cost materials for cryogenicapplications, a variety of approaches have been advanced. One employs aunique heat treatment to 6-8 percent nickel steels, the treatmentinvolving austenitizing, tempering above Ac cooling, and thentemperingat a temperature either above or below A0,. This approach islimited though, because of its inabilityto preserve the base plateproperties in the weldheat-affected zone, a property important instorage vessels where welding is, necessarily, extensively employed.Another approachinvolves modifying the alloying elements of the 6-8percent nickel steels. This improves the properties to some extent butdoes not offer the level of properties, especially transverse toughness,capable of meeting the requirements imposed at temperatures below 275F.Suffice to say, other proposals have been put forth, includingstrengthening the conventional 9 percent nickel steel such that thinnersections might be used. This too is not withoutobjection. i

In any case, it has now been discovered that satisfactory cryogenicproperties may be brought together in one steel provided the steelcontains correlated amounts of nickel, manganese, molybdenum, carbon,calcium, etc.,"as will be described more fully herein.

lt is an object of this invention to provide a steel having a yieldstrength above about 85 ksi and an ultimate tensile strength above about100 ksi together with Charpy V-notch toughness values of about 25,advantageously at least 30 ft.-lbs., or more in the longitudinaldirection and about 20, preferably at least 25 ft.-lbs., or higher inthe transverse direction at temperatures as low as -320F.

to 7.5 percent nickel, from about 0.4 percent to 2.8 percent manganese,the manganese and nickel being correlated so as to represent a pointwithin the area ACDEA of the accompanying drawing, from 0.01 to 0.5percent molybdenum, the molybdenum and nickel being interrelated as setforth hereinafter, calcium in a small. but effective amount to enhancetoughness and up to about 0.1 percent, from about 0.05 to 0.14 percentcarbon, up to about 0.5 percent silicon, up to about 0.1 percentcolumbium, the balance being essentially iron, the steel upon tempering,say at about l,ll0 to 1,200F., having a duplex microstructure comprisedof a martensitic matrix containing dispersed austenite. v

Nickel is essential in accordance herewith for strength and toughnesssince it acts as a stabilizer for the precipitated austenite dispersion.Amounts above 7.5 percent are unnecessary and a nickel range of 5.5

percent or 5.75 to 6.5 percent is highly satisfactory.

Manganese, as well as nickel, performs the role of lowering Ac, so thatan austenitic dispersion in a martensitic matrix is formed duringtempering. In addition to this and in seeking an optimum combination ofproperties, notably transverse toughness, the percentages of nickel andmanganese should be correlated to represent a point in the area ACDEA ofthe accompanying drawing and advantageously within the area BCDEB. Aswill be shown hereinafter, the effect of manganese on the toughnessproperties, both longitudinal and transverse, is related to the nickelcontent. A manganese range of 1.75 to 2.5 percent together with 5.5percent or 5.75 to 6.5 percent nickel affords good results.

The austenite dispersion is believed to be associated with or directlyresponsible for the outstanding cryogenie toughness in these nickelsteels. Manganese, however, even at the low concentrations in thesesteels, to wit, 0.4-2.8 percent, has been found to be beneficial mostly,if not completely, in respect of longitudinal toughness. Actually, it isdeemed detrimental to transverse toughness, particularly in amounts ofabout 1 per cent and higher, because it produces or contributes to astringer-like inclusion morphology, the stringers seemingly being mostlysulfides. Transverse toughness can, as is well known, be brought to alevel comparable I to that in the longitudinal direction by employingmore than the normal amount of cross rolling. This is expensive and thusunattractive (though full cross rolling is not excluded from theinvention). Alternatively, high purity-materials devoid of sulfur andother contaminants can be used as well as processing techniques designedto prevent or minimize the same. This too', also involves high cost andis thus not the most acceptable panacea.

To negate this aforedescribed detrimental effect on transverse toughnessit has been determined that cerv tain elements alter the inclusionmorphology from one Other objects and advantages will become apparentGenerally speaking, the present invention is directed to asteel'containing (by weight) from about 5 percent of stringers orstringer-like to globular form. Calcium is particularly beneficial inthis regard in amounts up to about 0.1 percent and in so doingcontributes to bringing about'an improvement in transverse properties. Aretained, calcium percentage of about 0.0l5, to 0.025 percent is quitebeneficial, although a range of 0.005 percent or 0.01 to 0.05 percent issatisfactory. Other additives, such as magnesium, barium, strontium,zirconium and rare earths (particularly cerium), may also be employed insimilar amounts. As a consequence of this aspect, the subject steel cancontain even up to as high as 0.04 percent sulfur although lower levels,up to 0.015 or 0.02 percent, are preferred for weldability.

Molybdenum is uscd for toughness and solid solution strengthening.Additions of only 0.1 percent markedly enhance impact properties, bothlongitudinal and transverse, and increase strength. But it should becorrelated to the nickel content, such that at percent nickel it is atleast 0.12 percent, at 5.5 percent nickel it is at least 0.08 percent,at 6 percent nickel it is at least 0.05 percent and at 7 percent nickelit is at least 0.01 percent. Above 7 percent nickel, molybdenum might beomitted but it is to be expected that properties would be lowered.(Interpolation can be used for intermediate percentages). Over the rangeof 5.5 to 6.5 per- I cent nickel a molybdenum level of 0.08 to 0.15percent is highly satisfactory.

Carbon has an important effect on toughness. Contrary to the generalrule in these types of steels that toughness increases as carbondecreases, it was found thatover the range of 0.05 to 0.13 percent or0.14 percent there is a maximum level of toughness at approximatelyabout 0.08 percent, i.e., about 0.07 to 0.09 percent. Toughnessdecreases gradually as the carbon content increases much above 0.08percent but decreases rather sharply as it is lowered from 0.08 percent.To achieve the desired toughnessthe carbon should be maintainedadvantageously above 0.05 percent, e.g., 0.06 percent or more, with the0.08 percent level being most preferred. Although up to 0.2 percentmight be tolerated, an upper limit of about 0.14 percent is beneficialsince higher levels also contribute to welding problems. I

1n carrying the invention into practice, the steels can be produced inaccordance with conventional procedures as those skilled in the art willappreciate. Both the melting steps and the casting and hot rolling stepsmay be carried out following conventional practices. The steel should bedeoxidized with a material such as aluminum, e.g., up to 0.15 percentaluminum, according to fine grain practice.' No unusual processingrestrictions are required. As to heat treatment, it is preferred thatthe steels be austenitized forl hour at about 1,500F., water quenched,and thereafter tempered for about 2 hours at about l,1251,l50F. followedby a water quench. The austenitizing temperature can be from l,425 to1,600F. and the tempering temperature can be from l,100 to 1,200F. Aircooling can be used. Other heat treatments may be utilized such as thedou- .ble normalize and temper (NN'l') treatment employed in connectionwith the standard 9 percent nickel steel. The important consideration istempering such that a two phase structure is achieved, to wit, atempered martensitic matrix with a relatively f ne and uniformdispersion of austenite, the austenite content being at least 3 percent,preferably at least 5 percent and most beneficially at least 10 percentby volume. The amount of austenite need not exceed 25 percent or 30percent.

For the purpose of giving those skilled in the art a betterunderstanding of the invention, the following description andillustrative data are given.

A number of steels were air-induction melted and cast into ingots. Highpurity raw materials were used. Electrolytic nickel and iron were usedtogether with ferromanganese, ferrosilicon, etc. Deliberate additions offerrophosphorus and ferrosulfide were made to simulate the purity incommercial 9 percent nickel steel. The charge was initially deoxidizedwith carbon and silicon-manganese. Final deoxidation was with aluminum.Calcium (calcium-silicon master alloy) was plunged into the melt afteraluminum deoxidation. All the steels were unidirectionally rolled toinch plate at 1,900F. The steels were then given a uniform austenitizingtreatment consisting of heating for one hour at 1,500F., waterquenching, tempering for 2 hours at 1,135F. and again water quenching.

Smooth-bar tensile specimens were cut from the longitudinal direction ofeach heat treated plate. Charpy V-notch impact specimens were taken fromboth the longitudinal and transverse directions of the plates. Thetensile tests were conducted at F. and the impact tests were conductedat 320F. Standard laboratory testing procedures were employed throughoutthis investigation.

The steels were of the following nominal composition (weight percent0.10 Mo, 0.25 Si, 0.12 C, 0.008 P, 0.015 S, 0.06 Al, 0.05 Cb and 0.015Ca, the nickel and manganese contents being given in Table I togetherwith tensile properties, including ir'npact toughness, and thepercentage of austenite.

TABLE l Austenite Alloy Ni Mn Content, Y.S. U.T.S. E1.

No. '71 7!- ksi ksi Long. Trans.

Y.S. yield strength (0.2; offset] U.T.S. ultimate tensile stren ksithousand pounds per square inch Table l reflects the desirability andadvantage in correlating the nickel and manganese contents. At the lowernickel levels (e.g., Alloy l), the manganese content should bemaintained at the upper part of its range to achieve the desiredtransverse toughness (Alloys 3 or 4). Alloy l is at best a marginalsteel and falls within the area ABEA of the drawing. in contrast, themore advantageous steels (Nos. 2-l 2) fall within the area defined byBCDEB. It might be added that at the higher nickel contents, toughnessis not nearly as sensitive to changes in the manganese content, thisbeing due to. the overpowering effect of nickel in this regard. Thenickel and manganese together with other austenite formers should bebalanced against the ferrite formers. This can be accomplished byprocedures well known to those skilled in the art (e.g., Schaefflersdiagram or modified versions thereof) so as to provide for amicrostructure containing preferably at least 5 percent by volume ofaustenite.

Although the steels in Table l nominally contained 0.05 percentcolumbium, this constituent is not essential and it is preferred that itbe omitted or at least be below 0.0l percent. A steel-similar to Alloy 7of Table I but to which columbium was not added exhibited a toughnesslevel of 41 ft.-lbs. in the longitudinal direction and 38"ft.-lbs. inthe transverse direction. This compares more than favorably with 30 and26 ft.-lbs. respectively. Tensile strengthwas lower but notdetrimentally so.

Carbon, as mentioned above and as can be seen from Table ll, has theunexpected effect of producing a maximum toughness, both longitudinaland transverse, as the level is increased-up to about 0.08 percent. Thetoughness decreases relatively rapidly at levels outside the range ofabout 0.07 to about 0.12 percent, particularly at the lower carbonlevels. The alloys of Table ll, apart from carbon, have the nominalcomposition (weight percent) of 6.1% Ni, 2.2% Mn, 0.1% Mo, 0.25%Si,'0.0l2% P,.0.0l6% S, 0.07% Cb and 0.015% Ca. it perhaps should bementioned that, all other factors remaining the same, the impactstrength of steels 13-1 6 would'be expected to be higher if the steelswere cent nickel steel. Welding tests have shown that the weld metal andheat-affected zone have adequate toughness at 320F.

The present invention contemplates articles, including structuralelements, made from the nickel steels described herein, andsubjected inuse to load at temperatures as low as 320F. Illustrative examples ofsuch articles include tanks, containers, reservoirs, vessels,'heatexchangers, and the like for producing, processing, storing and/ordistributing liquified gas, and associated equipment such as valves,pumps, piping, tubes, conduits, structural shapes for reinforcing orrestraining vessels, etc. The invention is particularly applicable forvessels containing such liquified gases as hydrocarbon gases, includingnatural gases, methane, propane, bu-

tane and other petroleum gases, and liquified nitrogen and liquifiedoxygen. I

As will be understood by those skilled in the art, the use of theexpression balance or balance essentially in referring tothe ironcontent of the subject steel does not exclude the presence of otherelements, such as those commonly present as incidental constituents,e.g., deoxidizing and cleansing elements, and impurities ordinarilyassociated therewith in small amounts which do not adversely affect thebasic characteristic of the steel. Silicon in amounts up to about 0.5percent, e.g.,.up to 0.3 percent, can be present. Phosphorus in normalamounts consistent with good steelmaking practice, for example, up toabout 0.025 percent may also be present.

Although the present invention has been described in conjunction withpreferred embodiments it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

1. An alloy steel adapted for cryogenic applications and characterizedby a yield strength above about 85,000 psi, Charpy V-notch toughnessvalues of at least about 25 and 20 'ft.-lbs. in the longitudinal andtranscolumbium-free. A

' TABLE .II

Y.S. Tensile Elong. CYN lmgacl Alloy c 0.2% Offset Strength in 1 N0.- Ipsi psi inch Long. Trans.

Table Ill shows the marked increase in transverse verse directions,respectively, down to temperatures of toughness caused by the additionof calcium or cerium. 5 5' about 320F., and good weldability, said steelconsist- The steels, in addition to calcium or cerium, nominallycontained 6.1% Ni, 2.1% Mn, 0.09% M0, 0.2% Si, 0.13% C, 0.06% Cb, 0.006%P and 0.013% S.

ing essentially of from about 5.5 to about 6.5 percent nickel, manganesepresent in anamount up to about 2.8 percent, the nickel and manganesebeing correlated The alloys of the invention are weldable usingsuchwires as the standard commercial wire used'for 9 perto represent a pointwithinthe area ACDEA of the accompanying drawing, at least 0.05 to 0.15percent moat least percent and up to about 25 percent by volume.

2. An alloy steel in accordance with claim 1 containing from5.5 to 6.5percent nickel from 1.75 to 2.5 percent manganese, from 0.05 to 0.12percent molybdenum, about 0.01 to 0.05 percent calcium, at least 0.06

I percent and up to 0.14 percent carbon, and the balance essentiallyiron, the austenite content being at least 10 percent.

22 UNITED STATES PA ENT OFFICE (IERTIFICATE OF CORRECTION s .s11,8-13 II m ia,3,11%21 Patent No.

Inventor(9 JOHN JOSEPH deBARBADILLO, II

It is certified that error appears in the abov e-identified patent ashown below;

and that'said Letters-Patent are hereby corrected a Col, 4, TABLE I/lastcolumn for "320 red .-320F. o

Col. .5, TAB LEtII, last column for 320F."r ad Col. 5, TABLE I11, lastcolumn for"'3 20F." read -320,F.--.

- Signe d ghdh'soeaied this 3r day of December 1974.

(SEAL) Attestz. I MCCOY M; GIBSON JR. t A c. MARSHALL ,DANN AttestingOfficer- Commissioner of Patents

2. An alloy steel in accordance with claim 1 containing from 5.5 to 6.5percent nickel from 1.75 to 2.5 percent manganese, from 0.05 to 0.12percent molybdenum, about 0.01 to 0.05 percent calcium, at least 0.06percent and up to 0.14 percent carbon, and the balance essentially iron,the austenite content being at least 10 percent.